Medical Physicist's Notes · Neutron Monitor
Six monitor positions andtwo thresholds at each.
A neutron-monitor deployment is more than just one monitor at the vault door. A typical cyclotron-facility neutron field map needs monitors at six locations across vault, adjacent workplaces and corridor. Each monitor carries two threshold levels — warning and action. This post walks the placement logic, the threshold-setting rationale and what triggers a re-survey of the field map.
Monitor placement
Six locations in a typical cyclotron / PET facility
| Location | Why | Priority |
|---|---|---|
| Vault interlock door (inside) | Reads the field operator faces at the moment of entry. Gates the safe-entry alarm. | Mandatory under AERB licensing |
| Vault interlock door (outside) | Reads the corridor field for personnel passing near the vault while production runs. Gates corridor evacuation if applicable. | Mandatory under AERB licensing |
| Adjacent radiopharmacy | Fast neutrons pass through standard hot-lab shielding. Documents staff workplace dose during runs. | Strongly recommended |
| Above the vault (next floor up) | Sky-shine neutrons can produce a measurable field on the floor above. Required when the floor above is occupied space. | Site-dependent |
| Cyclotron control room | Control-room neutron field is normally negligible but documented to confirm operator workplace compliance. | Documentation case |
| Target service / pneumatic transfer area | Target handling and pneumatic-transfer corridors carry activation neutrons during target service procedures. | Strongly recommended |
Source: AERB Safety Code for Medical Cyclotron Facilities; IAEA Safety Reports Series 47.
Threshold setting
Warning vs action — the two-level architecture
- Warning threshold: typically set at the workplace-dose-rate level that triggers an investigation but not an evacuation. Indicative value 5–10 µSv/h for cyclotron-adjacent workplaces; final value set against site-specific occupancy and the AERB licence.
- Action threshold: typically set at the level that requires immediate area evacuation or interlock-driven beam-off. Indicative value 20–50 µSv/h; final value site-specific.
- Vault-interior thresholds during beam-off (post-decay) are different — the vault is unoccupied during beam-on, so the vault-interior alarm is for the entry-decision window. Warning at the level acceptable for short-duration maintenance entry; action at the level that disallows entry.
- Threshold-setting paperwork (site-survey + licensing rationale) goes into the AERB facility-licence dossier alongside the type-test certificate and the calibration record.
Source: AERB Atomic Energy (Radiation Protection) Rules 2004; ICRP Publication 75.
When to re-survey
Five triggers for a field-map refresh
- Target service (chemistry change, target re-build): re-measure the post-beam-off decay curve at the vault-interior monitor. Activation-neutron tail may shift.
- Shield modification (any structural change to the vault): full re-survey, including all interior and adjacent-workplace monitors. The change can move the field map significantly.
- Beam-current uplift (if the cyclotron is upgraded to higher current): re-measure at all monitor positions. Workplace doses scale roughly linearly with average current.
- Adjacent-workplace renovation (hot lab refit, ward construction): re-measure at the adjacent monitor and consider adding new monitor positions if workspace layout changes.
- AERB inspection request: when the inspector asks to walk the field map, the monitor archive should already show the same map. Re-survey before inspection is a sign the map has been neglected.